1. Field of the Invention
This invention pertains to apparatus for measuring wheel alignment parameters of motor vehicles, and more particularly it pertains to apparatus which measures the deflection of a light beam which is reflected from a mirror or target placed upon a wheel of the vehicle.
2. Description of the Prior Art
In order to provide good performance from the steering apparatus of a motor vehicle and to promote tire life it is necessary that the front end of the vehicle be correctly aligned. To insure that the front end is correctly aligned the accurate measurement and adjustment of relatively small angles is necessary. Such measurements can and have been made with fairly simple instruments heretofore during a conventional front end or wheel alignment process. However, use of such prior art instruments is generally slow and time consuming if proper accuracy is to be obtained. In order to be effective, a wheel alignment machine should be both fast and accurate and it should be convenient for the average mechanic to use both for measuring and adjusting the alignment parameters.
In one type of prior art device, as shown in U.S. Pat. No. 3,758,213 to MacPherson et al and U.S. Pat. No. 3,630,623 to Schirmer, a light projector, mounted at the side of the vehicle, directs a beam of light to a mirror mounted on the wheel being measured. The beam is reflected from the wheel mirror to a viewing screen mounted adjacent to the light projector to indicate the alignment of the wheel. These prior art devices must be carefully aligned with the vehicle by moving either the vehicle or the alignment device in order to provide accurate measurements of the wheel alignment parameters.
In another type of prior art device, as shown in U.S. Pat. No. 2,700,319 to Carr, a source of light is mounted at one end of the vehicle parking space. A pair of narrow beams of light are projected across the end of the parking space where the beams are directed toward the wheels being measured by a pair of flat mirrors located near the front corners of the vehicle. The vehicle must be moved into a predetermined position in the parking space and accurately positioned in relation to the mirrors. This requires a special guide rack or excessive labor on the part of the operator. Any stress on the mirror supports may cause movement of the mirrors causing movement of the beams of light and inaccurate readings. Also, small inaccuracies in positioning the vehicle in relation to the mirrors can cause inaccuracies in the alignment of the wheels on the vehicle. Alternate methods are available which align the optical system with the vehicle by using a large movable mirror which is moved to direct the light beam to the wheel of the vehicle. However, this approach also requires several steps, and the accurate positioning of a relatively large mirror is a time consuming operation.